The lake is located in a flat basin with impermeable soil (a type of karst sinkhole called polje) between the mountain ranges of Mount Pomario (Veliki Javornik) and Mount Witches (Velika Slivnica).
The peculiarity of this body of water is that it is an ephemeral (or seasonal) karst lake, meaning that its volume depends on precipitation: its basin can empty completely in a short time, since in some places there are sinkholes capable of draining all the water, including that of the tributaries, into the cavity of the subsoil[3].
When it is in flood phase it invades the entire valley south of Circonio and becomes the largest lake in Slovenia in terms of size: its surface can in fact reach 38 km², while the altitude above sea level can vary from 546 to 551 meters. The maximum depth reaches 10 meters, but the average depth is only 1 meter, due to the irregularities of its volume.
Lake Circonio is formed by some tributary karst waterways, among which the main one is the Circonio torrent, which crosses the town of the same name; however, it does not have emissaries. The characteristics of this lake are similar to those of Lake Doberdò, in the province of Gorizia. The lake is an important natural reserve for many animal species, especially for the nesting of numerous bird species